Lead-acid batteries are mainly used in electric vehicles, in order to charge the lead-acid batteries reasonably, fast and efficiently, the lead-acid battery charger often uses a “three-stage” charge, that is, the constant current charging stage, the constant voltage charging stage and the tiny current charging (trickle charging) stage: the constant current charging is implemented in the first stage; when the voltage reaches the predetermined value, the constant voltage charging is implemented in the second stage and the current gradually decreases; then, the tin current charging is implemented in the third stage, the battery remains in a nearly fully charging state to continue the tiny current charging. For observing the charging state of the battery better, it is necessary to design a charging state indicating circuit for indicating the charging state of the battery.